The present invention pertains in general to methods and apparatus for the measurement of internal circumferences and in particular to methods and apparatus for measurement of the internal circumference of compliant rings.
Compliant rings, such as O-rings and circular cross section belts, are commonly molded out of a compliant material, often out of rubber. A particularly vexing problem arises from the fact that minor variations in the composition of the molded material can lead to varying amounts of post-molding shrinkage. Therefore, no matter how accurately the size of the mold is made it is necessary to measure samples of molded rings in order to determine whether the size of the ring falls within required tolerances.
Tolerances are particularly narrow for compliant rings which are used as sealing rings which are compressed within relatively narrow grooves about the circumference of containers in order to provide a liquid- or gas-tight seal. Tolerances for such rings commonly fall within the range of .+-.0.020 inches of circumference or less. Exceeding these tolerances may lead to rings which do not properly fit and which therefore may give rise to poor seals.
One approach to compliant ring measurement is to cut the ring and measure the length of the resulting linear configuration. However, deforming the ring in this way leads to stretching of the material along the original inner edge and compression of the outer edge which results in an inaccurate measurement of the length.
Another approach to measurement of compliant rings involves dropping the ring on a cone which is marked along its height to indicate its circumference. The accuracy of this method of measurement is limited by the ability of the operator to align the ring on the cone; by the ability of the operator to interpolate small distances between marks on the cone; and by variations in frictional forces between the cone and ring resulting in stretching of the ring.
Yet another approach involves drawing two concentric circles on transulucent paper corresponding to the ideal inner and outer circumferences of the ring and attempting to align the ring between the circles. Again the ability to align the ring between the circles and the ability to judge the degree of fit limit this approach.
All of the above approaches are slow, inaccurate, provide less than adequate support of and lead to elastic distortion of the rings being measured. Furthermore, the repeatability of measurements, especially important for large lot sampling, is limited.